There exist television systems of PAL (Phase Alternation by Line), SECAM (Sequential Couleur A Memoire) system, and NTSC (National Television Committee) system. These broadcasting systems differ in the number of scanning lines (625 lines/50 Hz for PAL and SECAM; 525 lines/60 Hz for NTSC) and in the frame frequency, intrinsically lacking the compatibility there among. Thus, in order to perform international broadcasting, program exchanges, etc., a technique of mutually converting the broadcasting systems has hitherto been developed and used in broadcasting stations, etc. Above all, the frame rate conversion is a process on the time axis and the motion repeatability after the conversion process may have a great influence on the image qualities, resulting in one of the most important techniques among the broadcasting system conversion techniques.
At present, a system conversion device using television digital processing detects and estimates a motion vector of an input image to perform a motion compensation of an interpolation image which is generated in accordance with the output frame rate, thereby carrying out an input/output frame rate conversion process (hereinafter, referred to as motion-compensated frame rate conversion process).
The scheme of the motion-compensated frame rate conversion process is as follows. First, from two or three consecutive images of a plurality of frames of an input image signal, a motion in the images is detected and estimated to obtain a motion vector of the input image (detection of motion vector). Known as this motion vector detection/estimation method are for example a gradient method, a block matching method, and a phase correlation method.
The thus obtained motion vector is then evaluated to select an optimum vector so that the length of the motion vector is adjusted in accordance with the input/output frame rate and that allocation is effected as an interpolation vector on an interpolation frame from the input image (allocation of interpolation vector). An image signal is finally allocated in accordance with the interpolation vector, from newly existing input frames time-axially anterior and posterior on the interpolation frame (generation of interpolation image) to perform frequency conversion of the output frames including the interpolation frame (image interpolation). As above, the input/output frame rate conversion is carried out through the processes comprising roughly of the motion vector detection, the interpolation vector allocation, the interpolation image generation, and the image interpolation.
The above motion-compensated frame rate conversion technique has originally been developed to convert image signals of different broadcasting systems, but recently become used also to improve the motion blur of a hold-type display device represented by a liquid crystal display device. In the hold-type display system, the state of light emission of each pixel is retained during approximately one frame period so that the impulse response of image display light has a time-sequential extension. Therefore, the time frequency characteristics deteriorate, which induces a reduction in the space frequency characteristics, causing a motion blur. That is, since the line of sight of a person smoothly follows a moving object, the image motion looks jerky and unnatural due to the time integral effect when the light emission time is long as in the hold-type display device.
By virtue of a higher frame rate of the input signal achieved by the motion-compensated frame rate conversion technique, the interpolation image signal is formed with a motion compensation so as to be able to improve a reduction in the space frequency characteristics causing a motion blur and to fully improve the motion blur disturbance of the hold-type display system (see, e.g., specification of Japanese Patent No. 3295437; and “Study on Dynamic Image Quality of Hold Emission Type Display by 8* CRT” by Shuichi Ishiguro and Taiichiro Kurita, IEICE Technical Report, The Institute of Electronics, Information and Communication Engineers (IEICE), EID96-4(1196-06), pp. 19-26). Such a technique of converting the frame rate (the number of frames) by interpolating an image between frames to improve the motion blur of the hold-type display device is called FRC (Frame Rate Converter), which has been put in practical use for the liquid crystal display device, etc.
FIG. 1 is a block diagram of a schematic configuration of an FRC drive display circuit in a conventional liquid crystal display device. As shown, the FRC drive display circuit is configured to include an FRC portion 10 that converts the number of frames of an input image signal by interpolating a motion-compensation image signal between frames of the input image signal; an active matrix type liquid crystal display panel 14 having a liquid crystal layer and an electrode for applying a scanning signal and a data signal to the liquid crystal layer; and an electrode driving portion 13 for driving a scanning electrode and a data electrode of the liquid crystal display panel 14 based on the image signal whose frame rate has been converted by the FRC portion 10.
The FRC portion 10 includes a motion vector detecting portion 11 that detects motion vector information from an input image signal, and an interpolation frame generating portion 12 that generates an interpolation frame based on the motion vector information acquired from the motion vector detecting portion 11.
The motion-compensated frame interpolation process is carried out using the motion vector information in this manner to increase the display frame frequency so that the display state of the LCD (hold-type display system) can approximate to the display state of the CRT (impulse type display system), enabling improvement in the image quality degradation attributable to a motion blur which may occur when displaying dynamic images.
Thus, as set forth hereinabove, execution of the motion-compensated frame rate conversion needs detection of a correct motion vector from consecutive front and rear frame images of an input image. The motion vector detection may however become difficult in the vicinity of the screen edges. The reasons therefor will hereinafter be described.
It is considered herein that a motion vector is detected between two frames, i.e., a preceding frame F1 and a current frame F2 of an input image, with the motion detection reference placed on the preceding frame F1. First, when an image enters from the outside of a screen as shown in FIG. 2, a partial image loss is present in the preceding frame F1, which intrinsically means that the motion detection reference is placed on this partial loss area. Therefore, an area indicated by (a) in an interpolation frame F12 generated has no interpolation vector allocated thereto and becomes indefinite.
As shown in FIG. 3, when an image leaves toward the outside of the screen, a partial image loss is present in the current frame F2, and hence the vector detection becomes infeasible at an area indicated by (b) in the preceding frame F1.
Thus, in either case, correct detection of the motion vector becomes difficult in the vicinity of the screen edges, so that an interpolation image generated as a result entails a degradation such as a disturbance or a distortion of the image.
By the way, ordinary motion vector detection includes a process of applying a proper filter to the detected motion vector. This is because when the vector detection is carried out on a block-to-block basis, smoothing with peripheral motion vectors to a certain degree will often ensure a visually less image degradation. Execution of such a filtering process may achieve a somewhat desired motion vector in the vicinity of the screen edges.
Nevertheless, one image of the input frame images for use in the interpolation partially lacks in the vicinity of the screen edges, with the result that it is difficult to carry out the same interpolation process as in the other areas (e.g., to perform linear image interpolation from the preceding and following frames in accordance with an interpolation vector), posing a problem also when the interpolation frame is generated.
A technique disclosed in Japanese Laid-Open Patent Publication No. 62-217784 for example is known as measures against the image degradation at the image edges in such the motion-compensated frame rate conversion process. The technique of Japanese Laid-Open Patent Publication No. 62-217784 obviates the problem occurring upon interpolation process among the above problems by adaptively switching to interpolation from only one frame having no partially lost image, i.e., unidirectional interpolation (translation of an input image using the interpolation vector) without performing interpolation from two, preceding and following frames, i.e., bidirectional interpolation (linear interpolation from images at both ends of the motion vector) in the vicinity of the screen edges when generating an interpolation frame image.